To delete a using memory and to substantially lengthen a training period.

A method for decoding maximum posteriori probability has a step of processing decoding by a sliding window method, a step of preserving probabilities β2L, β3L,.., for rears at an initial position of the other block obtained in a decoding process of each block this time in a storage unit 27 as the initial value of the probability for the rear of the other block in a next decoding process, and a step of starting the calculation of the probability for the rear of each block with the preserved initial value in the next time decoding process of probability calculators 23, 24 for rears.